Modern lubricating oils prepared from petroleum sources require multiple processing steps from the crude oil from which they are derived. Each of the processing steps is carefully controlled to achieve the required properties to meet modern lubricant needs and specifications. Lubricating base oils are the product from these processing steps. The base oils, in turn, provide the base ingredients that, when combined with generally smaller quantities of other materials, often termed “additives”, produce the lubricants that are the end use products for the process.
One challenge for the refiner in preparing base oils is to maintain a high selectivity of the desired product during each process step. Many of the process steps in producing lubricating base oil involves a chemical reaction, often in the presence of at least one catalyst. The more selective each catalyst is for the reactions that occur in a particular process step, the higher the amount of feed is converted into the desired product in the process step. Other products that are formed during the process step are generally of lower value than the desired product. Improving one or more process steps often includes changes to the catalyst, feed or process conditions that results in higher selectivity to the desired product, and thus ultimately a higher yield of lubricating base oil.
A lubricating base oil process using a petroleum based feedstock generally produces a range of lubricating base oils, differentiated at least by boiling range and by viscosity. Lower viscosity base oil products tend to dominate the product slate from a particular process. In contrast, higher viscosity base oils are often more difficult to make. Heteroatoms such as sulfur and nitrogen tend to be concentrated in heavier petroleum fractions, and the processes to remove them tend to reduce the yield of high viscosity products. Heavier petroleum fractions also tend to concentrate aromatics and other low viscosity index molecules; upgrading these fractions to achieve a high viscosity index has the same negative impact on yield of the high viscosity products.
A number of methods have been proposed for producing high quality base oils having a high viscosity. For example, U.S. Pat. No. 7,776,206 describes a distillation method for producing a lubricant bright stock. The goal remains of developing new catalytic processes for producing a high viscosity lubricating base oil at high yields.